1. Field of the Invention
The present invention relates to hermetic compressors and refrigerating-freezing devices.
2. Background Art
In recent years, hermetic compressors used in freezer devices such as domestic fridge-freezers are strongly desired to have low electric power consumption. This type of conventional hermetic compressor provides high volumetric efficiency by making the outer shape of the piston unique so as to reduce the sliding loss between the piston and the cylinder. One such example is disclosed in Japanese Patent Unexamined Publication No. 2004-169684.
The above-mentioned conventional hermetic compressor will be described as follows with reference to drawings. FIG. 9 is a longitudinal sectional view of the conventional hermetic compressor, and FIG. 10 is a perspective view of a piston in the hermetic compressor.
As shown in FIGS. 9 and 10, the hermetic compressor includes airtight container 1 having motor element 4 and compression element 5 driven thereby, which are suspended by a plurality of springs 25. Motor element 4 includes stator 2 and rotor 3. Airtight container 1 contains oil 6 at its bottom.
Compression element 5 includes crankshaft 10 having main shaft 11 and eccentric part 12 eccentric to main shaft 11. Main shaft 11 has rotor 3 fixedly fitted thereto, and an oil pump (not shown) having an opening in oil 6.
Compression element 5 further includes block 20 above motor element 4. Block 20 has substantially cylindrical cylinder 21 and bearing 22 for supporting main shaft 11. Compression element 5 further includes piston 30, which is reciprocably inserted in cylinder 21 and connected to eccentric part 12 via connection means 41.
Piston 30, which is composed of top end surface 31, skirt end surface 32, and outer peripheral surface 33, has piston pin bore 38 parallel to main shaft 11. Piston pin 36 is inserted through piston pin bore 38 and connected to connection means 41.
The hermetic compressor having the above-described structure operates as follows.
When supplied with electric power, motor element 4 rotates rotor 3, and hence, crankshaft 10. At this moment, the eccentric rotation of eccentric part 12 of crankshaft 10 is transmitted to piston 30 via connection means 41, making piston 30 reciprocate in cylinder 21.
With the reciprocation of piston 30, refrigerant gas 40 in airtight container 1 is suctioned into intake muffler 45 and then into cylinder 21. At the same time, low-pressure refrigerant gas 40 is drawn into airtight container 1 through a cooling system (not shown). Refrigerant gas 40 suctioned into cylinder 21 is compressed, and discharged again to the cooling system.
When the hermetic compressor is in operation, piston 30 reciprocates in cylinder 21. As a result, oil 6 pumped up by the oil pump is supplied to the sliding part between cylinder 21 and outer peripheral surface 33 of piston 30, thereby lubricating and sealing the sliding part.
In the above-described conventional structure, however, when piston 30 moves from the bottom dead center to the top dead center in the compression stroke, top end surface 31 of piston 30 is subjected to the compressive load of the refrigerant. This causes crankshaft 10 to be pushed strongly in the direction opposite to piston 30 via connection means 41, and then to be bent. As a result, piston 30 is greatly inclined in the vertical direction, without becoming posturally stable.
Moreover, in the above-described conventional structure, the vertical inclination of piston 30 with respect to cylinder 21 can be controlled only between the end of top end surface 31 and the end of skirt end surface 32 by the clearance between outer peripheral surface 33 of piston 30 and cylinder 21.
As a result, piston 30 is greatly inclined, increasing the clearance between outer peripheral surface 33 of piston 30 and cylinder 21, and hence causing more refrigerant to leak from the top-dead-center side to the bottom-dead-center side of piston 30 and then through the clearance. This results in a reduction in the refrigeration capacity of the compressor.
Such a reduction in the refrigeration capacity increases the sliding loss particularly in a slow rotation of 23 rps or so, thereby decreasing the volumetric efficiency of the compressor, and hence increasing the electric power consumption of the refrigeration cycle.
In the case of using a refrigerant R600a, piston 30 is required to have a large outer diameter. This makes the refrigerant leak more easily and increases inclination fluctuation of piston 30 in the vertical direction, causing the sliding loss due to vibration and collision, thereby remarkably decreasing the volumetric efficiency. The term “inclination fluctuation” of piston 30″ means that piston 30 is posturally unstable because crankshaft 10 is bent as a result of top end surface 31 of piston 30 being subjected to the compressive load of the refrigerant.